Method to integrate buoyance in a mooring line and the mooring line resulting therefrom.

Abstract

A method to make a mooring line buoyant is presented. The method provides for circumferentially bending float sheets around a mooring line. The float sheets are provided with grooves that allows for bending of the otherwise rigid flow sheet and results in a filled collar with little free space in between. Thereafter the bent float sheet is permanently held to the mooring line by means of a jacket. Optionally the float sheet may be temporarily fixed to the mooring line by staples or tapes. The resulting buoyant mooring line has less than 10% of free space remaining in the float sheet.

Description

Method to integrate buoyance in a mooring line and the mooring line resulting therefrom.DescriptionTechnical Fields

[0001] The invention relates to a method to make a mooring line buoyant over its entire length or over a part thereof. The invention also relates to the product resulting from said method.Background Art

[0002] Mooring lines are used to dock vessels, to hold floating structures and more and more also to anchor floating offshore wind turbine structures. Mooring lines can be made of steel wire or can be made from synthetic material yarns such as polyamide, polyester, aromatic polyamide, polypropylene, and high molecular weight polyethylene. While these synthetic materials generally are lighter per unit of length for the same breaking load compared to steel rope, they still have a density above that of sea water i.e. they have negative buoyancy and sink when unwound into the sea.

[0003] For most applications this is not an issue but in some cases it is beneficial that the mooring line is floating e.g. when installing or connecting a vessel to a platform. Moreover, any additional buoyancy be it over part of the length of the mooring line or to make it completely floating will improve the handling of the rope and can reduce the top tension of the mooring line, as - part of - the weight of the line is carried by the water.

[0004] Certain ways exist to improve buoyancy for example by attaching discrete buoys that are externally attached to the ropes. However, this requires on site installation and only locally imparts buoyancy.

[0005] Alternatively there has been suggested in EP 4 223 926 A1 to helically wind block-shaped buoyancy elements mounted on an elongated carrier around the synthetic rope that are subsequently held by a braided outer jacket.

[0006] In another embodiment US 10633790 B2, functional elements are helically wound around a rope. The functional element can be a buoyancy means.

[0007] In both embodiments, the helical winding prevents to impart a large local buoyancy as the width of the functional elements or the block shaped buoyancy is limited due to the way they are applied.Disclosure of Invention

[0008] It is therefore an object of the invention to provide a method to impart buoyancy to a mooring line. More in particular the method allows to obtain maximal local buoyancy with a minimum amount of material. Furthermore the method results in a compact, local buoyancy support of the mooring line.

[0009] According a first aspect of the invention a method to impart buoyancy to a mooring line is presented.

[0010] In one step a mooring line is provided. Such mooring line is made of polymer monofilaments or multifilamentary yams made of polyester, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), aliphatic polyamides, such as PA6 or PA6.6, or aromatic polyamide (Aramid), polyethylene (PE), such as high molecular weight polyethylene (HMPE), polypropylene (PP), and / or mixtures thereof.

[0011] Yams are twisted into rope yams, rope yams are combined into rope strands, rope strands are formed into a sub-rope made of three, four to seven rope strands twisted together. Ten to hundred sub ropes are arranged parallel - forming the load bearing core - and held together with a non-load bearing braided jacket of polymer yams, or helically wrapped with a tape like woven or non-woven structures. Other structures such as a filter layer or a polymer coating over part or the complete length of the mooring rope can also be included. All this forms the mooring line that has a substantially circular cross section with a diameter ‘d’.

[0012] In another step a float sheet is provided that is made of float sheet material. The float sheet is preferably a solid closed foam sheet made of light and strong polymer such as polyvinylchloride (PVC) or ethylene-vinyl acetate (EVA) polyethylene. Another material is rigid polyurethan (PUR) foam. Alternatively hollow glass micro-size (diameter between 1 to 1000 pm) beads or microbubbles or microballoons can also be used as fillers in an epoxy resins. The float sheet has a density or mass per unit volume ofbetween 45 kg / m3to 450 kg / m3. Lower than 45 kg / m3conflicts with the required compressive strength, while more than 450 kg / m3does not give sufficient buoyancy. Preferably the density is at least 60 or at least 80 or even above 100 kg / m3. For buoyancy the density is best below 400 kg / m3, or below 300 kg / m3or even below 200 kg / m3. Density is measured as per ISO 845. The float sheet is of cuboid shape and has a thickness ‘t’ expressed in millimetre. The thickness of the sheet is the smallest dimension of the cuboid shape.

[0013] The compressive strength of the float sheet material determines to what depth the float sheet can resist the water column pressure. As roughly every 10 meter below sea water level the pressure increases with 1 bar (0.1 MPa), a float sheet with a compressive strength of 9 MPa (90 bar), can withstand a depth of 900 meter. Thereafter it will collapse, crush and buoyancy is lost. This by itself does not need to be a problem: it can also be a desired property of the mooring line, to lose buoyancy from a certain depth onward. In an advantageous implementation of the method, the float sheet material is selected to have a compressive strength expressed in megapascal that one hundredth of the intended crush depth expressed in meters. Hence a float sheet that crushes, collapses at 8 MPa (80 bar, 800 m), 7 MPa (70 bar, 700 m), 6 MPa (60 bar, 600 m) down to 2 MPa (20 bar, 200 meter) or even down to 1 MPa (10 bar, 100 m) can be desired. Compressive strength is measured according ASTM D1621.

[0014] Special of the float sheet is that it is provided with multiple grooves at a first side of the float sheet. The grooves reach to near to, close to the second side of the float sheet, the second side being opposite the first side. However, the grooves do not cut the sheet. Preferably more than 5% of the width ‘t’ of the sheet remains, or even 10% or 15%, or up to 50% remains intact. More will not work anymore in the step following. The grooves are oriented lengthwise of said float sheet i.e. the grooves define the length dimension. The width dimension is perpendicular to the length dimension and the thickness. The width dimension is equal to or about equal to the circumference of a circle with diameter ‘d+2xf i.e. the width is equal to irx(d+2xt), wherein ‘IT’ represents Archimedes’ constant.

[0015] The grooves can be formed in the float sheet material by any of the following techniques:- Cutting, sawing, grinding grooves in a cuboid plate made of float sheet material. The disadvantage is that material is lost, however this may remain below 12%, depending on the depth of the groove (less deep groove, more material saved). Normally the plates will be cut from a bigger cuboid.- Moulding a float sheet in final shape. This can be done but is more cumbersome as the mould is expensive. Possibly a mould wherein removable inserts are arranged can be used.- Use 3D printing to add material to an otherwise thin floating plate.

[0016] The grooves have a groove gap or width measured perpendicular to the longitudinal direction that monotonically diminishes from the opening at the first side to zero near to the second side of the float sheet i.e. at the bottom of the groove. Preferably the groove gap diminishes linearly from the opening width at the first side to zero near to the second side of the float sheet.

[0017] The grooves can have a straight ( V) or skewed (| / ) - V - shape, or they can be U-shaped or they may have the shape of a half lune shape.

[0018] When adding all the openings at the first side of the float sheet this must add up to about to or equal to 2xrrxt or the circumference of a circle with radius ‘t’.

[0019] The number of grooves ‘N’ is 10 or more, preferably 15 to 20, but not larger than 60. The number of grooves determines how well the float sheet can follow the outer diameter of the mooring line. The number of grooves determines the bottom groove angle that cannot be less than 360 / N degree, more preferably is 400 / N, but can go up to 720 / N.

[0020] In one way of implementing the method float sheets are prepared in advance with a number of grooves. Once the diameter of the mooring line to be provided with buoyancy is known, the width of the floating sheet is adapted by selecting the needed number of grooves to surround the ropes and cracking off the not necessary grooves.

[0021] In a following step, the float sheet is circumferentially bent around the mooring line, with the first side - with the grooves in - towards the mooring line. The grooves are aligned with the axis of the mooring line. Float sheet material is rigid, but by the thin remaining bridge (5% of the thickness of the sheet or more) that forms at the second side of the float sheet, the float sheet can be bend around the mooring line while remaining integral. The application of the float sheet can be done by intermediately stopping the line applying the jacket, locally applying the floating sheet and fixing it temporarily and then again start the line. Alternatively this can be done on a platform moving with the mooring line. The application of the floating sheet i.e. bending around the line and temporarily or permanently fixing it can be done by hand or by means of a robot.

[0022] As a last step the bent float sheet is fixed by a jacket to the mooring line. The jacket completely surrounds, holds the float sheet in place. The jacket can be a braided jacket comprising a braid of yarns, possibly the same as the ones that cover the mooring line. As the diameter of the mooring line with circumferential floating sheet locally increases, the lay angle of the braid - the angle between the laid fibres and the axis if the mooring line - may increase compared to the lay angle outside of the floating sheet. Alternatively, the jacket may be extruded over the float sheet. As another alternative, the bent floating sheets may be held in place by means of a heat shrink sleeve. To that purpose, prior to applying the floating sheets, the mooring line may be inserted in a series of heat shrinkable sleeves.

[0023] As an intermediate step, after bending the float sheets around the mooring line and before applying the jacket, the circumferential ends of said float sheet are permanently or temporarily held to one another with staples, preferably hard polymer staples. Alternatively connectors can be used that hold the two ends together. Another alternative is that the ends are provided by interlocking protrusions - much like the protrusions of a jigsaw puzzle - that lock into one another.

[0024] Another way of holding the bent float sheet in temporarily or permanently in place that can be used in conjunction with the previous method or on its own is to hold the sheet with a tape or a bandage or a scrim.

[0025] In a further preferred method, the float sheets are provided with incisions that extend from the second side towards close to the first side, the incisions being perpendicular to the lengthwise direction of the float sheet. These incisions can be straight cuts, or can be groove like. At the incisions, the float sheet is very week as it is only held at the top of the teeth at the first side of the sheet. These incisions serve as preferred breaking paths when the mooring line - provided with the floating sheets - is being bent. When the mooring line is e.g. rolled onto a reel, the floating sheets will neatly break at the incisions and allow for a shorter bending radius.

[0026] According a second aspect of the invention a self-buoyant mooring line is presented as produced, manufactured, made according any one of the above presented methods. The buoyancy elements are integrated into the outer jacket of said buoyant mooring line.

[0027] A more specific embodiment of the self-buoyant mooring line, comprises one or more float sheets with grooves, the float sheets being arranged circumferentially around the mooring line, wherein less than 10% of free space remains in the float sheet. With ‘free space’ is meant that when a cross section perpendicular to the axis of the mooring line is made, the area not taken up by the float sheet - i.e. that is accessible to air or water as a consequence of the presence of the grooves - between the inner, first side of the float sheet and the outer, second side of the float sheet multiplied by the length of the float sheet is less than 10% of the volume between the inner side and the outer side.

[0028] In a further specific embodiment the self-buoyant mooring line is provided with float sheet material that collapses from a specific dept onward. As maximum a depth of 900 meter is suggested such that the float sheet material must have a compressive strength of at least 9 MPa. However, this limit can be set at will by selection of the material: for 800 meter a compressive strength of 8 MP is needed, the need compressive stress asexpressed in MPa, being the depth divided by a hundred.Brief Description of Figures in the Drawings

[0029] FIGURE 1 illustrates the method according the first aspect of the invention.

[0030] FIGURE 2 is a schematic illustration of a float sheet as needed for the method.

[0031] FIGURE 3 is an illustration of a cross section of a finished self-buoyant mooring line.

[0032] The hundred digits in the reference numbers refer to the figure number and the unit and tens number refer to identical or similar features across figures.Mode(s) for Carrying Out the Invention

[0033] The method will now be explained in more detail by the aid of FIGURE 1 . The top accolades indicated 102 to 108 define the different steps in the method 100. In a first step 102 a mooring line 101 of diameter ‘d’ 180 mm is provided. The mooring line is of conventional make and comprises 12 sub-ropes held together and parallel by a braided jacket. The sub-ropes consist of three strands twisted together, the strands consist of rope yarns twisted together. The rope yarns are made of polyethylene. The rope has a linear mass density of 28 kg per meter.

[0034] In a second step 104 a float sheet 110 is provided. FIGURE 2 shows this float sheet 210 in more detail. The float sheet has a first side 228 and a second side 226 The float sheet 210 has a thickness ‘t’ of 27 mm. The float sheet material 220 has a volume mass density of 130 kg / m3and a compressive strength of about 2.0 MPa, meaning that the buoyancy will survive 200 meter of water depth, but not much more. The sheet has a length ‘L’ of 1200 mm and a width ‘W of 735 mm that is irx(d+2xt). It is made of closed cell expanded PVC foam. There are 20 grooves 222 made in it by sawing up to a depth of 22 mm implying that 5 mm - indicated as ‘at’ - or 18.5% of the sheet flow material remains as a bridge between grooves. The groove bottom angle is 22°. At 223 an incision has been made perpendicular to the lengthwise direction ‘L’ of the float sheetextending from the second side towards the first side. Small bridges at the first side remain.

[0035] In a next step going forward from 104 to 105 the float sheet 110 is bend around the mooring line 101 as indicated by the arrows. The grooves in the float sheet close, and the float sheet becomes a sleeve 112 held around the mooring line.

[0036] At 106 the float sheet is temporarily held by a tape 114, before it enters the braider section 108. In the braider section a jacket is braided by two interwoven yarn bundles 116 that hold the float sheet in place. The final rope diameter is 250 mm. Over a total rope length of 213 meter, 203 meter is covered with float sheets. The mass per meter over the total length has increased to 29.8 kg / m due to the presence of the float sheets and the jacket.

[0037] Figure 3 shows a cross section 300 of the resulting self-buoyant mooring line at one of the floating sheets. The mooring line 301 has a diameter ‘d’. It is surrounded by the floating sheet 312 that is circumferentially bent around the mooring line. The grooves have collapsed to lines 322. The total diameter of the assembly has become d+2xt. The floating sheet 312 is held in place by the jacket 318.

[0038] The resulting self-buoyant mooring line has in air a full rope mass of about 6350 kg, that is a weight force of 62 300 N. In the water it floats with its centre at around 30 mm under water level. When fully pulled under water the self-buoyant mooring line experiences an upward force of 116 newton per meter. When held under water over its complete length at the ends, the self-buoyant mooring line will take the curve of an inverted catenary.

Claims

Claims1 . A method of manufacturing a self-buoyant mooring line , said method comprising the following steps:- Providing a mooring line with diameter ‘d’,- Providing a float sheet made of float sheet material, said float sheet having a thickness ‘t’, said float sheet having multiple grooves in the first side of said float sheet, said grooves reaching to near the second side of said float sheet, said second side being opposite to said first side, said grooves being oriented lengthwise of said float sheet, said float sheet material having a mass density betweeen 45 to 450 kg / m3- Circumferentially bending said float sheet around said mooring line, with said first side towards said mooring line, with the grooves of said float sheet aligned with the axis of said mooring line,- Providing a jacket over the bent float sheet, thereby obtaining a self-buoyant mooring line.

2. The method according to claim 1 wherein the grooves reach to 50% up to 5% of the thickness of the float sheet from said second side of the float sheet.

3. The method according to claim 1 or 2 wherein said float sheet has a width, perpendicular to the lengthwise direction of said float sheet, said width matching the circumference of a circle of diameter ‘d+2xt’.

4. The method according to any one of claims 1 to 3 wherein the total opening of the grooves at the first side of the float sheet is about 2rrt.

5. The method according to any one of claims 1 to 4 wherein the number of grooves is at least ten.

6. The method according to any one of claims 1 to 5 wherein the width of the grooves monotonically diminishes from the opening width at the first side to zero near to the second side of the float sheet.

7. The method according to any one of claims 1 to 6 wherein the grooves are straight or skewed V-shaped, or are U-shaped, or are half lune shaped.

8. The method according to any one of claims 1 to 7 wherein, after circumferentially folding said float sheet around said mooring line, the circumferential ends of said float sheet are held to one another with staples, connectors, or interlocking provisions.

9. The method according to any one of claims 1 to 8 wherein, after circumferentially bending said float sheet around said mooring line, said float sheet is circumferentially held with a tape or a bandage or scrim.

10. The method according to any one of claims 1 to 9 wherein said float sheet has one or more incisions, said incisions extending from said second side towards close to said first side, said incisions being perpendicular to the lengthwise direction of said float sheet.11 .The method according to any one of claims 1 to 10 wherein said float sheet is made by removal of material, form moulding of material, or addition of material.

12. The method according to any one of claims 1 to 11 wherein the float sheet material selected to have a compressive strength expressed in megapascal that is one hundredth of the intended crush depth expressed in meter.

13. A self-buoyant mooring line as manufactured according to any one of the methods as per claim 1 to 12, said self-bouyant mooring line comprising a mooring line and buoyancy elements, said buoyancy elements being integrated into the jacket of said buoyant mooring line.

14. The self-buoyant mooring line according to claim 13 that comprises one or more float sheets having grooves, said float sheets being arranged circumferentially to said mooring line, wherein less than 10% of free space remains in the float sheet.

15. The self-buoyant mooring line according to any one of claims 13 to 14 wherein the mass density of the float sheet material is from 45 to 450 kg / m3

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